Torsion–rotation analysis of OH stretch overtone–torsion combination bands in methanol

We report rotationally resolved spectra of jet-cooled methanol for the OH stretch overtones, ${\text{2v}}_1$ and ${\text{3v}}_1,$ and for the torsional combinations, ${\text{2v}}_1{\mathrm{+v}}_{12},$ ${\text{2v}}_1{\text{+2v}}_{12},$ ${\text{3v}}_1{\mathrm{+v}}_{12},$ and ${\text{3v}}_1{\text{+2v}}_{12}.$ The spectra are obtained by direct excitation from the vibrational ground state with an infrared laser pulse. Population in the resulting upper state levels is detected by infrared laser assisted photofragment spectroscopy (IRLAPS). Global fits of the spectra to the Herbst Hamiltonian yield the torsional and rotational parameters, including F, ρ, $V_3,$ and $V_6,$ for each OH stretch excited state. For each quantum of OH stretch excitation, we find that the torsional barrier height $V_3$ increases by $\text{40.9±1.9\hspace{0.05em}}{\mathrm{cm}}^{\mathrm{-1}}$ and the torsional inertial F decreases by $\text{0.89±0.02\hspace{0.05em}}{\mathrm{cm}}^{\mathrm{-1}}.$ With reference to ab initio calculations, we explain the increase in $V_3$ in terms of changes in the electronic structure of methanol as the OH bond is elongated. For ${\mathrm{\Delta v}}_{12}\text{=1}$ we observe only transitions with $\text{ΔK=±1,}$ and for ${\mathrm{\Delta v}}_{12}\text{=2}$ we observe only $\text{ΔK=0.}$ We present a Franck–Condon model to explain these apparent selection rules and the overall pattern of intensity.